The characteristics of second-harmonic-generation from femtosecond optical pulses are discussed. Analytical solutions for the harmonic pulse shape, and conversion efficiency in non-depletion condition are presented. In time domain coupled equations in general condition are solved by numerical method. Phase mismatching influnce on the harmonic pulse shape and conversion efficiency is analysed. Experimental study of extraoavity SHG in LBO crystal from 80fs optical pulses, produced by 0PM laser, is reported, the experimental results agree with the theory very well.